Thin-film transistor (TFT) liquid displays (LCDs) have been widely used in areas such as personal notebook computers, portable televisions and video camera monitors. The requirement in high quality and high density of the image pixels on the LCDs becomes more and more difficult to satisfy. The conventional process of fabricating thin-film transistor for LCDs using low electron mobility amorphous silicon (a-Si) material can no longer meet the requirement of a high quality display. It is necessary to use material with high electron mobility in fabricating thin-film transistors for LCDs. It is also important that similar thin-film transistors can be fabricated for a large size LCD. Therefore, polycrystalline silicon is an appropriate material for such an application.
When a polycrystalline silicon thin film transistor is used as a switching device for the LCD, electrical voltage difference V.sub.DS between the drain and the source can be as high as .+-.5 volts. The voltage difference mostly drops in the depletion area near the drain. A very large electric field is introduced in the depletion area. The electrons coming from the source are blocked by the depletion area. A large amount of electron holes are generated due to the collision into the depletion area by these electrons. Therefore, it is very likely for the electrons located in the drain area to recombine with the electron holes on the other side of the depletion area because of the energy gap provided by some defects in the material and the tunneling effect. Consequently, there can be a current leakage when the thin film transistor is turned off and the image quality of the LCD is degraded.
To solve the current leakage problem, the electrical field in the depletion area near the drain has to be reduced in order to minimize the possibility of the electron and hole recombination caused by the tunneling effect. One commonly used approach is to add an undoped offset area between the gate and the drain or a lightly doped drain area. Under this circumstance, most of the voltage difference drops in the added area. By adjusting the length of the added area, the electrical field in the area can be changed in order to reduce the current leakage. For thin-film transistors in an LCD, it is necessary that drain and the source are symmetrical and exchangeable so that the storage capacitance in the liquid crystal can be charged and discharged symmetrically to change polarity. Therefore, an undoped offset area or a lightly doped area near the source also has to be fabricated.
In making a large size high quality LCD, all the thin-film transistors should have the same characteristics. However, the undoped offset area or the lightly doped drain/source area may have different length which affects the threshold voltage, the sub-threshold swing, and the field effect carrier mobility of the thin-film transistor. This also results in changes in the characteristics of the transistor. In practice, it is not easy to maintain the same length for the two offset areas or lightly doped areas near the source and drain area. Trying to manufacture offset areas or lightly doped areas with the same length for a large number of thin-film transistors on the LCD is another difficult challenge.